1. Field of the Invention
The field of this invention is electromagnetic field fiber optic sensors.
2. Background Discussion
In the past decade, many different types of fiber optic sensors have been developed. The physical effects that these sensors monitor include temperature, pressure, electric field, magnetic field, acoustic acceleration, rotation, and velocity. These sensors have been developed due to the increased sensitivity, geometric versatility, and potential low cost that fiber optics affords.
The most sensitive type of fiber optic sensor is the "interferometric" type in which the output power from a laser is divided by a beam-splitter and both of these laser beams are coupled into separate fiber optic cables. Fiber optics which are of similar length and construction, but which differ slightly in that a coating has been applied to the outside surface of one of the fibers which makes the coated fiber sensitive to the physical effect to be monitored, may be used with such an interferometer.
The exposure of the coated fiber optic to the physical effect, such as an applied magnetic field, causes small changes in the coated fibers' length which are not seen in the uncoated fiber. When the two laser beams output by the coated and uncoated fibers are mixed together by a second beam-splitter they will either add constructively or destructively causing the light intensity input to the photo detector to vary. It is by monitoring the output signal from the photo detector that the physical effects can be determined.
The key in developing an interferometric fiber optic sensor is in finding the appropriate coating which sensitizes one of the fibers to the effect that is to be measured. Sometimes the other fiber needs to be coated as well to desensitize it to the same effect. Interferonmetric sensors are so sensitive that they may be affected by several physical effects, such as temperature or acoustic vibrations. Thus, it is sometimes necessary to isolate the uncoated or "reference" fiber in an environment as to minimize extraneous effects. Sometimes it is necessary for both fibers to be routed through nearly the same path to help subtract these extraneous effects.
With an appropriate coating, the fiber optic may be sensitive to microwave radiation, and these sensors may find numerous commercial and military applications. For example, this type of sensor with its high sensitivity can be used to detect distant microwave sources of either natural or man-made origin. Phased arrays may be used to determine the direction of arrival and the distance to the microwave source. Two different types of coating may have some utility in making the fiber sensitive to microwave energy.
Electrostrictive coatings, such as Polyvinylidene Fluoride (PVF.sub.2), have been used in the past to make inteferometric electric field sensors and phase modulators. This material has a high sensitivity which yields a minimum detectable field strength of 6 micron volts per meter. A diagram describing two different configurations of a PVF.sub.2 sensor is shown in FIG. 1a. In the first configuration, PVF.sub.2 material is coated onto a piece of single mode fiber. The coated fiber is then heated slightly (50.degree.-70.degree. C.) and then exposed to a large external electric field. The electric field forces the polymer molecules to align along the electric field direction. This happens faster and at lower field strengths when the PVF.sub.2 is slightly heated. After the sensor is cooled off the molecules remain polarized along the original electric field direction. The PVF.sub.2 sensor is then "poled" and is now electrostrictive. In its original coated state, the PVF.sub.2 molecules are randomly oriented and thus have no electrostrictive properties.
The second configuration of a PVF.sub.2 sensor shown in FIG. 1b has the PVF.sub.2 radially poled. In order to accomplish this, a thin metallic coating must be applied to the fiber optic before the PVF.sub.2 material is applied. An electric field is then established between the thin metal coating on the fiber, and an external concentric circular metal electrode.
Another commonly used technique is to cover the outer surface of the PVF.sub.2 material with a thin metal coating. The radial poling is useful when the PVF.sub.2 sensor is to be used as a modulator since there are two electrodes (inner and outer metal coatings) to couple electrical signals into. The linearly poled sensor is useful for external fields.
As an electric field is applied across the poled PVF.sub.2 sensor, it causes the molecules of the plastic to stress the fiber optic. This stress, through the photoelastic effect, causes the local index of refraction of the fiber to increase or decrease slightly. Since the uncoated reference fiber does not see any effect from the EM field when the sensor and reference fibers laser beams mix together, their phase-fronts add up constructively or destructively. This leads to intensity fluctuations which constitute the monitored signal.
A fundamental limitation to the use of PVF.sub.2 or any interferometric sensor to monitor EM fields in the microwave range is the transit time of light through the fiber optic. At 1 GHz, the electric field reverses its sign every 0.5 ns. If the fiber sensor is accurately responding to the external electric field, the sign of the induced phase shift also reverses. Thus, the phase shift accumulated during the first half of the EM fields cycles is subtracted off during the second half. For fiber sensors greater than 10 centimeters in length, the resulting output phase of the sensor will never grow larger than that accumulated by a length of fiber ##EQU1## where Fm=Frequency of applied electric field and c/n=Speed of light in the fiber,
A sensor of Lm would be able to respond to frequencies below Fm and slightly above Fm. But this sensor would have low gain making measurements of weak or distant sources difficult.